1. Field of the Invention
The present invention relates to a thermal type infrared ray imaging device and a fabrication method thereof in which a plurality of cells are provided for each pixel.
2. Related Background Art
Recently, as a thermal type infrared ray image sensor in which a cooling apparatus is unnecessary, a bolometer type made of vanadium oxide using a micromachine technique and a pyroelectric type made of BST (Barium-Strontium-Titanate) type have been sold commercially. These products has a heat sensitive part for absorbing infrared ray to raise temperature, a support leg for thermally separating the heat sensitive part from a silicon substrate, a horizontal address line for selecting pixels, and a vertical signal line.
This type of infrared ray image sensor absorbs the infrared ray radiated from objects at the heat sensitive part, and detects temperature rise of the heat sensitive part by resistance variation and capacitance variation. Because of this, the support pillar for supporting the heat sensitive part has a structure in which cross section is small and length is long, in order to enhance adiabatic effect.
However, when the length of the support leg is long, the infrared ray is absorbed to convert the infrared ray into heat, thereby decreasing area of the heat sensitive part for converting the heat into an electric signal. That is, the length of the support leg and the area of the heat sensitive part are trade-off.
When the area of the heat sensitive part decreases, dead area of image area increases. When a picture of a faraway small object is taken, a spot focused on an image area becomes about 14 μm of diffraction limit. Because of this, when this spot of 14 μm is focused on non-sensitive area, no picture can be taken. Accordingly, as shown in FIG. 24, Japanese patent Laid-open publication No. 209418/1998 and so on disclose a structure in which a heat sensitive part 123 is divided into a photothermal conversion part 125 and a heat sensitive conversion part 124, and area of the heat sensitive conversion part 125 is enlarged. In FIG. 24, reference number 121 is a substrate, reference number 122 is a bonding pad, reference number 126 is a hollow part, reference number 127 is a support leg for supporting the heat sensitive part 123 on the substrate 121, reference number 128 is a vertical signal line.
It is possible to estimate emissivity of an object under test, by measuring intensity of a plurality of infrared rays radiated from the objects. Therefore, it is possible to easily identify materials, and to precisely measure absolute temperature. A method of performing the entire processings by image is disclosed in Japanese Laid-open publication No. 23261/1997 and Japanese Laid-open publication No. 188407/2000. A cooling type image sensor of HgCdTe or GaAlAs/GsAs system quantum well structure type is used in these documents. The sensor has a feature in which it is possible to easily obtain lamination structure, and an absorption waveband is very narrow. It is possible to selectively absorb the infrared ray of each waveband in each layer of the lamination layer.
However, in a non-cooling type infrared ray image sensor, for example, Japanese application No. 201400/2001 discloses a method in which detection pixels of two wave lengths are alternately disposed for every line or every column. In the sensor disclosed in this document, a non-sensitive area increases at each waveband, and it becomes difficult to take a picture of a faraway small object.
Conventionally, it is possible to estimate emissivity of the object under test by measuring a plurality of infrared rays emitted from objects. Therefore, it is possible to identify a material of the object under test, and to precisely measure the absolute temperature. However, there was a problem in which the non-sensitive area at each waveband increases, and it became difficult to take a picture of the faraway small object.